The invention relates to mechanical expansion mine roof bolts, and more particularly to expansion shell assemblies thereof which can be reliably anchored into strata surrounding a bolt hole.
A variety of mine roof bolt assemblies are known in the art. They usually include a bolt of from three to six feet in length, a support plate through which the roof bolt extends and an expansion shell assembly threaded onto a threaded upper end of the roof bolt. A mine roof bolt hole is drilled, usually perpendicular to the surface of the mine roof, and the expansion shell assembly installed on the upper end of a roof bolt is inserted into the roof bolt hole such that the support plate abuts and supports the mine roof. The roof bolt then is tightened, causing the expansion shell assembly to expand, thereby anchoring the entire assembly, including the roof bolt, to the mine roof strata and forcing the support plate upwardly against the mine roof. Exemplary of the prior art are U.S. Pat. Nos. 4,095,431; 4,100,748; 4,292,244; and 4,295,760.
However, when conventional mechanical expansion type mine roof bolts have been inserted into a roof bolt hole, and the nut at the lower end of the roof bolt then is torqued to tighten it, the anchor shell sometimes fails to engage the wall of the bolt hole. In this event, the expansion shell simply rotates with the roof bolt and fails to become anchored therein. Such failures of anchoring are quite frequent, and can be caused by an oversized bolt hole, improper adjustment of the relationship between the wedge nut and the expansion wings of the anchor assembly, or failure of the wedge nut to advance down the threaded shank of the bolt. Furthermore, if the bolt threads or wedge nut threads are rusted, or if there is grit in the threads, or if they are damaged or inadequately lubricated, the anchor shell may fail to engage the roof bolt walls adequately to prevent it from rotating with the bolt. The wedge nut then will not advance into the expansion wings and spread them to cause anchoring.
When the expansion shell assembly initially fails to engage the wall of the bolt hole, the bolt assembly typically is maneuvered around in the hole, and further attempts are made to engage the anchor shell with the wall of the bolt hole, but if this fails, the bolt and anchor shell assembly must be withdrawn. The shell assembly must then be carefully adjusted to hopefully achieve adequate engagement with the bolt hole wall. Sometimes this requires several attempts to achieve proper adjustment, and sometimes, especially if the wedge nut is very tightly engaged with the roof bolt due to rusted or damaged threads, it is necessary to simply withdraw the assembly and replace it with another bolt anchor assembly. These inconveniences are time consuming and costly.
Therefore, it is an object of the invention to provide an improved expansion shell anchor assembly for use with a mine roof bolt to assure adequate engagement of the anchor shell assembly with the roof bolt to cause advancement of the wedge nut during torquing of the roof bolt, thereby anchoring the expansion shell without undue inconvenience.
It is another object of the invention to provide an expansion shell anchor assembly which is inexpensive and yet reliably engages a bolt hole to assure anchoring despite rust or damage of threads of the bolt or wedge nut.